1. Field of the Invention
This invention relates to the processing and curing of composite propellants. More specifically, the present invention relates to the use of small amounts of liquid dialkyl tin catalysts with polyurethane binders to lower viscosity and improve processing and curing properties for high-solids composite propellants.
2. Technology Review
A common problem with composite propellant compositions is a lack of oxidizer for efficient combustion. One proposed solution is to use oxygenated polymeric binders, such as polyethers, polypropylene oxide, and polybutylene oxide in a high-solids system, to supplement conventional oxidizers in the propellant formulation. Regrettably, such polymers do not cure well using conventional curing agents and catalysts such as IPDI (isophorone diisocyanate) and TPB (triphenyl bismuth).
Polyurethane binders composed of polypropylene oxide and isocyanate curatives have been used as binders in composite propellants. In practice, the solid component incorporated into such polymer systems never exceeds about 85%. Composite propellant compositions having high solids content are desirable because they have better performance characteristics, such as specific impulse (pounds of force generated per pound of propellant). Unfortunately, above 85% solids content, the viscosity becomes too high to process.
It was found that raising the mixing and curing temperature above about 135.degree. F. (60.degree. C.), lowers the viscosity enough to enable processing of the composite propellant. However, high temperature mixing and curing requires expensive facilities and equipment and induces stresses and strains in the propellant grain upon cooling to ambient temperatures. In addition, for systems utilizing ammonium nitrate as an oxidizer, ammonium nitrate undergoes a phase change at temperatures above about 90.degree. F. (32.degree. C.) in which the ammonium nitrate crystals tend to grow and swell. This is extremely undesirable because it can destroy the shape and performance properties of processed composite fuel grains. Additives, such as metal oxides and nitrates, are known to stabilize the ammonium nitrate phase change, but they are expensive and unreliable. Moreover, since such additives are inert, they decrease the propellant's performance characteristics.
It will be appreciated that there is a need in the art for methods and compositions for processing and curing composite propellants having high solids content at ambient temperatures.
Such methods and compositions for processing and curing high-solids composite propellants are disclosed and claimed herein.